Germ

Currently there are four types of germs in the game: Food poisoning, Slimelung, Floral scents, and Zombie spores.

Multiplication
Germs may cover tiles, items, buildings, and duplicants. Different materials may affect germs differently: for example, Slimelung can only thrive (multiply) in Polluted Oxygen and Slime, all other materials will slowly or quickly kill it.

Germs on an item, building or Duplicant experience some of the effects of the surrounding gas or liquid. For example, Slime stored in Chlorine will quickly become disinfected fully, despite Slime causing Slimelung to multiply.

Each phase of matter has a different upper limit on germs per entity (tile, item etc.). This limit is multiplied by entity's mass. Once that limit is reached, the germs become "Overpopulated" and start dying out at a speed dependent on degree of overpopulation.

Floral Scents don't multiply, but they are constantly generated by Bristle Berries and Buddy Buds.

Spreading
Fluids (gases and liquids) constantly mix, exchanging germs with fluids of the same phase. Gases and liquids do not exchange germs with each other directly, although Polluted Water will emit Polluted Oxygen with a part of its germs on it.

Solid tiles do not exchange germs, but a solid tile on which germs multiply (currently only Slime with Slimelung) will infect direct neighbors.

Only one type of germ can exist in any kind of tile, so if you put a Buddy Bud (which make Floral Scents) near some Polluted Oxygen (particularly from Slimelung infected Slime), they'll carry Floral Scents instead of Slimelung.

Duplicants will exchange germs with buildings and items by interacting with them. Germs on duplicant's surface are not dangerous yet. They may be removed from the duplicant by usage of a Wash basin or Shower. This will transfer the germs partly into the Polluted Water produced by the building and partly onto the building itself. Climbing a Ladder counts as interaction.

Buildings that change resources into other resources generally do not destroy germs. For as long as the building outputs something, this output will usually retain all the germs of the inputs. Notably, a Deodorizer will not destroy Slimelung in Polluted Oxygen - it will simply transfer it to its products. This is generally not a problem as most germs can only thrive on very specific substances.

Metal Refinery gains surface germs after each refining job, when it uses coolant containing germs.

Infection
Germs can reside on a duplicant or in the duplicant. Germs on a duplicant do not cause any direct harm. Only germs in a duplicant can cause infections. There are two ways for a duplicant to become infected: breathing in infected gas or eating infected Food.

A duplicant who breathes Oxygen or Polluted Oxygen will absorb all of the germs in the consumed gas. This is the most common way a duplicant gets infected with Slimelung, Allergic Reaction (only for dupes with the Allergic trait, and they come in contact Floral Scents) or the Zombie Spores.

A duplicant who eats infected food will absorb all of the germs on the food item, typically transferred there by the harvester, cook, or the duplicant eating with dirty hands. Slimelung will quickly die on solids, but Food Poisoning readily multiplies on food.

Whenever a duplicant comes in contact with infected gas (only for slimelung, zombie spores, and floral scents) or consumes (food poisoning) infected food, they have a per-cycle chance to contract an illness depending on the germ and their resistance (listed below).

If they fail to fight off the infection, the duplicant gains a Disease and suffer from its symptoms until they recover after some time, or taking the appropriate care and medicine. A duplicant can't gain a second disease from germ infection while already diseased. Note that Heat Stroke or Hypothermia counts as a disease.

Floral scents has a 100% chance to cause allergic reaction, but only in dupes with the allergies trait. If a dupe lacks this trait they have a 0% chance of getting allergic reaction, instead they get the smelled flowers buff, which gives them some stress loss.

Illness
Here are the current per-cycle chances a duplicant has to contract an illness based on their resistance after an exposure to the germs.

The duplicant resistance is 0 by default, with below modification factors:
 * Vitamin Chews: +0.5
 * Immuno Booster: +1
 * Duplicants with Biohazardous trait: -1
 * Duplicants with Germ Resistant trait: +1

Treatment
Treatment depends on the disease. Allergic Reaction and Food Poisoning can be completely cured with Allergy Medication and Curative Tablets respectively, and needs no treatment from a doctor, aside from fabricating said medicine using an Apothecary. Allergy medication will also prevent them from getting allergies again for 1.2 cycles.

Slimelung is cured with Medical Packs, which is also produced in the Apothecary but can only be used in the Sick Bay, and both need doctors.

The Zombie Spores can be cured in Disease Clinics, though they're expensive to cure as they need Steel and Sun Nymph Eggs to produce Serum Vials.

Reduction
Dupes can be instructed to disinfect buildings (Tidy) when the germ numbers reach a limit (10,000 by default). There is no way to explicitly disinfect anything else, save for the Medicine Buildings that allow Dupes to wash themselves.

Germs naturally die out when the conditions are not favorable. The conditions differ for each germ.

Items placed in Chlorine is fully disinfected in a cycle (all germs have a dying rate of 100%/cycle in Chlorine). This includes Slime, which continuously generates germs. Liquids can be disinfected by storing them in a Liquid Reservoir immersed in Chlorine (see Guide/Disinfect Food Poisoning).

Tips

 * Although the tooltips of Plastic Tiles and Plastic Ladders say that plastic is antiseptic, they have no in-game effect.

History
Germs have been introduced as part of the Outbreak update, and have been revamped in the Quality of Life III update.